A variety of sensors have been developed for measuring current in a current carrying cable, such as current in a high voltage electrical distribution system. Optical current sensors 30 based on the birefringence of various materials, and the Faraday effect, from the magnetic field, and optical voltage sensors based on the Pockels effect, from the voltage field, are known in the art. Optical current sensors, that use fiber optic cable that surround the current carrying cable, although they may have a suitable dynamic range, require opening the current carrying cable at installation. Hence they are expensive and cumbersome to install.
Optical current sensors utilizing a magnetic concentrator with bulk optical sensors, (as opposed to fiber optics), in an airgap are also known in the art. One such embodiment is discussed in an article titled “Use of Dual Frequency Excitation Method to Improve the Accuracy of an Optical Current Sensor,” by Shuping Wang, et al, SPIE meeting, August, 2009.
Airgaps within powder core magnetic material stabilize the temperature sensitivity of the magnetic material. Such stabilization, with respect to laminated magnetic core structures, is discussed in the publication “Gapped Magnetic Core Structures,” by Guenter B. Finke, Magnetic Metals Corporation, Camden, N.J. 08101.
Blake, U.S. Pat. No. 6,166,816, describes the use of one light source for a combined fiber optic current and voltage sensor. It is, however, difficult to make use of the current sensor disclosed. The electric utility company can use it during a new set up or take apart the current carrying cable for installation.
Ishiko et al., U.S. Pat. No. 4,999,571, describes a clamp-on optical current and voltage sensor. The sensor must be attached to the line when the voltage to the line is off. The crystal used in the current sensor is a garnet crystal, which is temperature sensitive. The sensor also uses a quarter-wave plate in connection with the voltage sensor and such quarter-wave plate is also temperature sensitive. In Ishiko, the current carrying cable is not firmly held by the device until the U-shaped, magnetic core is in its closed position.
Bjorn, U.S. Pat. No. 7,068,025, teaches a simplified sensor, a small glass rod lying on the current carrying cable. Based on the Faraday effect, rotation of the plane of polarization of polarized light in the glass rod is proportional to the intensity of the magnetic field surrounding the cable. The strength of the magnetic field surrounding the conductor is in accordance with the level of electric current passing through the conductor. The Bjorn patent teaches a method in which the sensor samples only one locality and only for a short distance along the conductor.
C. V Temple et al., U.S. Pat. No. 2,709,800, teaches a power line fault detector that allows mechanical adjustment of the airgap of a concentrator and inductive coupling for detecting various levels of current. This sensor may only be used for the detection of fault currents.
Bosselmann et al., U.S. Pat. No. 5,963,026, discloses two Faraday elements or crystals for two 5 different measurement ranges of current in order to achieve a higher dynamic range. This adds to the complexity and the cost.
The prior art teaches various devices and methods for measuring the current in real time in a current carrying cable using optical sensors. However, the prior art does not teach an economical, simple sensor assembly design for accurate measurements across a large dynamic range, sensitivity and bandwidth, that is capable of being installed on the cable without disturbing the operating function of the cable. The present disclosure fulfills these and other needs and provides further related advantages as described in the following summary.
A need also exists for a small, compact current sensor that is highly sensitive to a magnetic field of interest but highly insensitive to unwanted magnetic fields, such as from nearby current carrying cables.